feat: context engine plugins, scaffolding, and pricing fixes (#6)

* feat: add 4 context engine plugins

- topic-memory: keyword clustering for topic-aware memory recall
- episodic-memory: conversation segmentation and cross-session recall
- user-profile: persistent user profiling from conversation patterns
- context-decay: time-based memory decay with reinforcement dynamics

All plugins use the ingest/after_turn hook protocol with stdin/stdout JSON.

* chore: add plugin scaffolding, update docs and templates

- Add plugin.toml template with {{NAME}} placeholder
- Add new-plugin Makefile target with hooks/ scaffolding
- Update plugins/README.md with all 10 plugins
- Update README.md stats (10 plugins, 220+ models)
- Add Plugin checkbox and checklist to PR template
- Add Plugin to issue template content type dropdown
- Fix CONTRIBUTING.md: last_verified is recommended, not required

* fix: correct model pricing and remove deprecated entries

- openrouter/gemma-2-9b-it: fix pricing from 0.0 to 0.03/0.09 per M tokens
  (free variant correctly stays at 0.0)
- github-copilot: remove deprecated copilot/gpt-4 model entry
  (GPT-4 retired in favor of GPT-4o for Copilot)

* docs: annotate kimi-coding as membership-gated

Kimi Code CLI uses quota-based membership model (not per-token billing).
Free tier has limited weekly requests; underlying model is K2.5.
Pricing kept at 0.0 consistent with other subscription providers
(chatgpt, github-copilot) but with explanatory comments.

* style: fix trailing newline in github-copilot.toml

* fix: correct Moonshot/Kimi model pricing from official sources

All 5 models had incorrect pricing:
- moonshot-v1-8k: 0.10/0.10 → 0.20/2.00
- moonshot-v1-32k: 0.30/0.30 → 1.00/3.00
- moonshot-v1-128k: 0.80/0.80 → 2.00/5.00
- kimi-k2: 2.00/8.00 → 0.60/2.50
- kimi-k2.5: 2.00/8.00 → 0.45/2.20

Sources: platform.moonshot.ai/docs/pricing/chat, costgoat.com, getmaxim.ai

* feat: add MiniMax M2.7 and M2.7-highspeed models

Released 2026-03-18, MiniMax's latest flagship text model.
10B activated params, 200K context, 128K output, tool use, streaming.
Pricing: $0.30/$1.20 per M tokens (input/output).

Added to both international (minimax.io) and China (minimaxi.com) providers.

Makefile

@@ -9,6 +9,7 @@ AGENTS_DIR    := agents
 HANDS_DIR     := hands
 INTEGRATIONS_DIR := integrations
 SKILLS_DIR    := skills
+PLUGINS_DIR   := plugins
 PROVIDERS_DIR := providers
 
 # ── Validation ────────────────────────────────────────────────────────────────
@@ -80,6 +81,18 @@ new-skill: ## Scaffold a new skill (usage: make new-skill NAME=my-skill)
 	@echo "Created $(SKILLS_DIR)/$(NAME)/skill.toml"
 	@echo "Next: edit the skill.toml to fill in TODO placeholders."
 
+.PHONY: new-plugin
+new-plugin: ## Scaffold a new plugin (usage: make new-plugin NAME=my-plugin)
+	@if [ -z "$(NAME)" ]; then echo "ERROR: NAME is required. Usage: make new-plugin NAME=my-plugin"; exit 1; fi
+	@if [ -d "$(PLUGINS_DIR)/$(NAME)" ]; then echo "ERROR: Plugin '$(NAME)' already exists."; exit 1; fi
+	@mkdir -p "$(PLUGINS_DIR)/$(NAME)/hooks"
+	@sed 's/{{NAME}}/$(NAME)/g' "$(TEMPLATES_DIR)/plugin.toml" > "$(PLUGINS_DIR)/$(NAME)/plugin.toml"
+	@touch "$(PLUGINS_DIR)/$(NAME)/hooks/ingest.py"
+	@touch "$(PLUGINS_DIR)/$(NAME)/hooks/after_turn.py"
+	@touch "$(PLUGINS_DIR)/$(NAME)/requirements.txt"
+	@echo "Created $(PLUGINS_DIR)/$(NAME)/ with plugin.toml and hooks/"
+	@echo "Next: implement the hook scripts and fill in TODO placeholders."
+
 .PHONY: new-provider
 new-provider: ## Scaffold a new provider (usage: make new-provider NAME=my-provider)
 	@if [ -z "$(NAME)" ]; then echo "ERROR: NAME is required. Usage: make new-provider NAME=my-provider"; exit 1; fi
@@ -102,4 +115,5 @@ help: ## Show this help
 	@echo "  make new-hand NAME=my-hand"
 	@echo "  make new-integration NAME=my-service"
 	@echo "  make new-skill NAME=my-skill"
+	@echo "  make new-plugin NAME=my-plugin"
 	@echo "  make new-provider NAME=my-provider"